I am completely new to LaTeX, I installed MacTeX, and when I try to typeset I get an error saying

/usr/pdflatex does not exist. Perhaps TeXLive was not installed or was removed during a system upgrade. If so, go to the TeXShop web site and follow the instructions to (re)install TeXLive. Another possibility is that a tool path is incorrectly configured in TeXShop preferences. This can happen if you are using the fink teTeX distribution.